Viewing optical system and imaging apparatus using the same

ABSTRACT

The invention provides a viewing optical system positioned between a viewing plane as a virtual plane and an eye point. The viewing optical system comprises, in order from the viewing plane side, a cemented lens in which at least one negative lens and at least one positive lens are cemented together and one positive lens, and satisfies the following condition (1). 
       −8&lt; r 3/ f &lt;−0.2  (1) 
     Here r3 is the radius of curvature of the lens surface positioned in the cemented lens and nearest to the viewing plane side, and f is the focal length of the whole viewing optical system.

This application claims benefit of Japanese Application No. 2008-132721filed in Japan on May 21, 2008, the contents of which are incorporatedby this reference.

BACKGROUND OF THE INVENTION

The invention relates to a viewing optical system, and an imagingapparatus using the same.

Patent Publication 1 shows a viewfinder that uses an aspheric surface tocorrect it for distortion with fewer lenses.

Patent Publication 1: JP(A) 5-215974

SUMMARY OF THE INVENTION

The present invention provides a viewing optical system positionedbetween a viewing plane as a virtual surface and an eye point,characterized by comprising, in order from said viewing plane side, acemented lens in which at least one negative lens and at least onepositive lens are cemented together, and one positive lens, andsatisfying the following condition (1):

−8<r3/f<−0.2  (1)

where r3 is the radius of curvature of the lens surface positioned insaid cemented lens and nearest to said viewing plane side, and

f is the focal length of the whole viewing optical system.

The present invention also provides an imaging apparatus characterizedby comprising an imaging device, an image display device adapted todisplay an image, a controller adapted to convert image informationobtained from said imaging device into signals displayable on said imagedisplay device, and a viewfinder adapted to guide an image displayed onsaid image display device to a viewer's eye, wherein the above viewingoptical system is used for said viewfinder.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent form the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is illustrative of Example 1 of the inventive viewfinder.

FIG. 2 is illustrative of Example 2 of the inventive viewfinder.

FIG. 3 is illustrative of Example 3 of the inventive viewfinder.

FIG. 4 is illustrative of Example 4 of the inventive viewfinder.

FIG. 5 is an aberration diagram for the viewfinder of Example 1.

FIG. 6 is an aberration diagram for the viewfinder of Example 2.

FIG. 7 is an aberration diagram for the viewfinder of Example 3.

FIG. 8 is an aberration diagram for the viewfinder of Example 4.

FIG. 9 is illustrative in construction of a digital camera that is oneexemplar of the inventive imaging apparatus.

FIG. 10 is illustrative in construction of the inventive imagingapparatus applied to a silver-halide camera.

FIG. 11( a) is illustrative of Condition (2), and FIG. 11( b) isillustrative of Condition (3).

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some embodiments of the inventive optical system are now explained. Inwhat follows, it should be noted that an eye point E refers to aposition (the position of a virtual stop S) where the farthest off-axislight beam leaving a viewing plane D passes full through the virtualstop S of φ4. At this position, the diameter of the farthest off-axislight beam is substantially in coincidence with the diameter (φ4) of anaperture in the virtual stop S. It should also be noted that an eyepoint distance EP refers to a distance from the lens surface located inthe viewing optical system (eyepiece lens) and nearest to an eye pointside to the eye point (see FIG. 11(a)).

It should be noted that in FIG. 11( a), the given position is off theposition of the eye point (virtual stop): the eye point distance is 20mm longer. However, the given position may be in coincidence with theposition of the eye point (virtual stop). When the given position isdifferent from the eye point position, there is a difference in widthbetween the upper and lower light beams at that eye point position. Whenthe given position is identical with the eye point position, the widthsof the upper and lower light beams are going to be equal at the eyepoint position.

The viewing optical system here is suitable for use on a viewfinder. Inthe viewfinder, specific objects such as a field stop, ground glass oran image display device are located on a position of the viewing planeD. However, the viewing optical system itself has none of the specificobjects at the position of the viewing plane D. In the viewing opticalsystem, therefore, the viewing plane D becomes a virtual surface.

The viewing optical system here is interposed between the viewing planeas a virtual surface and the eye point. This viewing optical systemcomprises, in order from the viewing plane side, a cemented lens inwhich at least one negative lens and at least one positive lens arecemented together, and one positive lens. With such arrangement, theviewing optical system here enables chromatic aberrations to be wellcorrected at the negative and positive lenses in the cemented lens. Theviewing optical system here includes one positive lens in addition tothe cemented lens. With one such positive lens, therefore, it ispossible to gain an adequate eye point distance and a wide angle offield.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (1).

−8<r3/f<−0.2  (1)

Here r3 is the radius of curvature of the lens surface positioned in thecemented lens and nearest to the viewing plane side, and

f is the focal length of the whole viewing optical system.

By the satisfaction of Condition (1), it is possible to makesatisfactory correction of aberrations while the optical system is keptcompact. As the lower limit of −8 to Condition (1) is not reached, thereis an increasing load of the positive lens positioned nearest to the eyepoint side on correction of aberrations. This is not preferable, becausefield curvature in particular or the like goes worse.

As the upper limit of −0.2 to Condition (1) is exceeded, it causes theradius of curvature of the viewing plane side of the cemented lens togrow tight or become small. This is not preferable, because sphericalaberrations and coma go worse.

More preferably for correction of aberrations, the upper limit ofCondition (1) should be set at −6, and the lower limit should be set at−4.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (2).

0°<εh<20°  (2)

Here εh is an exit angle (°) of the farthest off-axis chief ray on theviewing plane provided that the farthest off-axis chief ray is theoutermost of off-axis chief rays that intersect the optical axis of theviewing optical system at the given position, and the given position is20 mm spaced away from the lens surface located in the viewing opticalsystem and nearest to the eye point side toward the eye point side (seeFIG. 11( a)). Note here that the clockwise direction from a referenceposition is plus, and the counterclockwise direction is minus.

By the satisfaction of Condition (2), it is possible to keep the optimumeye point distance so that the size of the optical system can bereduced.

As the lower limit of 0° to Condition (2) is not reached, it causes therange of light rays from the viewing plane to grow wide. This is notpreferable because the outer diameter of the optical system grows large.

As the upper limit of 20° to Condition (2) is exceeded, it causes therange of light rays from the viewing plane to turn too inward. This isnot preferable because to make sure the eye point distance, the opticalsystem must have a longer total length.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (3).

0<Enx/Y1<40  (3)

Here Enx is a distance from the viewing plane to an entrance pupil, and

Y1 is the height of a given off-axis chief ray on the viewing plane,provided that the given off-axis chief ray is defined by a chief raycorresponding to an angle of field of 30° of off-axis chief rays thatintersect the optical axis of the viewing optical system at the givenposition, and the given position is 20 mm spaced away from the lenssurface located in the viewing optical system and nearest to the eyepoint side toward the eye point side (see FIG. 11( b)).

Being short of the lower limit of 0 to Condition (3) is not preferablebecause the whole length of the optical system grows long.

Exceeding the upper limit of 30 to Condition (3) is again not preferablebecause there is coma produced.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (4).

−0.68<f12/f<−0.15  (4)

Here f12 is a combined focal length of lenses between the viewing planeand the negative lens, and

f is the focal length of the whole imaging optical system.

When there is a lens between the viewing plane and the cemented lens,f12 is going to represent the combined focal length of that lens and thenegative lens in the cemented lens. When there is no lens between theviewing plane and the cemented lens, f12 is going to represent the focallength of the negative lens in the cemented lens.

As the lower limit of −0.68 to Condition (4) is not reached, it causesthe power of the whole optical system to grow too strong. This is notpreferable because the contour of the optical system grows large.

As the upper limit of −0.15 to Condition (4) is exceeded, it causes thepower of the whole optical system to become too weak. This is notpreferable because not only does the whole length of the optical systemgrow long, but also chromatic aberrations cannot well be corrected.

For the viewing optical system here, it is preferable to satisfy thefollowing conditions (5) and (5)′.

1.7<n<2.2  (5)

1.7<n′<2.2  (5)′

Here n is the refractive index of the lens located in the cemented lensand nearest to the viewing plane side, and

n′ is the refractive index of the lens located in the cemented lens andnearest to the eye point side.

As the lower limit of 1.7 to Conditions (5) and (5)′ is not reached, theradius of curvature grows tight. This is not preferable because there iscoma produced.

Exceeding the upper limit of 2.2 to Conditions (5) and (5)′ is notpreferable because correction of field curvature, etc. is difficult.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (6).

|n−n′|<0.15  (6)

It is preferable to satisfy Condition (6) because monochromaticaberration produced at the cementing surface is reduced, and adequatecorrection of colors can well be done.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (7).

13 mm<EP<40 mm  (7)

Here EP is the eye point distance that is a distance in mm from the lenssurface located in the viewing optical system and nearest to the eyepoint side to the eye point.

As the lower limit of 13 to Condition (7) is not reached, there is noseparation occurring between the center light beam and the peripherallight beam at the positive lens nearest to the eye point (for instance,the aforesaid one positive lens). This is not preferable because it isdifficult to offer a sensible tradeoff between center performance andperipheral performance.

Exceeding the upper limit of 40 to Condition (7) is not preferable,partly because the positive lens nearest to the eye point grows large,and partly because the amount of aberrations produced of the peripherallight beams grows large.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (8).

13.5 mm<f<45 mm  (8)

Here f is the focal length of the whole viewing optical system.

Being short of the lower limit of 13.5 to Condition (8) is notpreferable because the eye point distance becomes short.

Exceeding the upper limit of 45 to Condition (8) is not preferablebecause the whole optical system length grows long.

For the viewing optical system here, it is preferable to satisfy thefollowing condition (9).

0.08<tan θ×EP/f<1.6  (9)

Here θ is the maximum angle of field,

EP is the eye point distance, and

f is the focal length of the whole viewing optical system.

As the lower limit of 0.08 to Condition (9) is not reached, it causesthe angle of field to become small, and the eye point distance to becomeshort as well. This is not preferable because difficulty is encounteredin separation between the light beams near the center axis and at theperiphery, resulting in difficulty in offering a sensible tradeoffbetween center performance and peripheral performance.

As the upper limit of 1.6 to Condition (9) is exceeded, it causes thefocal length of the whole viewing optical system to become short. Thisis not preferable because the eye point distance and the power(refracting power) of the positive lens nearest to the eye point are illbalanced with the result that peripheral performance is likely todeteriorate.

For the viewing optical system here, it is preferable to satisfy thefollowing conditions (10) and (11).

0.85<f1/f<3  (10)

0<(r−r′)/(r+r′)<30  (11)

Here f1 is the focal length of the one positive lens,

f is the focal length of the whole viewing optical system,

r is the radius of curvature of the lens surface of the one positivelens on the viewing plane side, and

r′ is the radius of curvature of the lens surface of the one positivelens on the eye point side.

As the lower limit of 0.85 to Condition (10) is not reached, it causesthe focal length of the one positive lens to become short, producinglarge aberrations. This is not preferable because difficulty isencountered in correcting the produced aberrations.

As the upper limit of 3 to Condition (10) is exceeded, it causes theouter diameter of the viewing optical system to grow large. It alsocauses the amount of aberrations produced at the cemented lens to growlarge. This is not preferable because difficulty is encountered inoffering a sensible tradeoff between correction of chromatic aberrationsand correction of monochromatic off-axis aberrations.

As the lower limit of 0 to Condition (11) is not reached, the curvatureof the lens surface of the one positive lens on the viewing plane sidegrows tight. In this case, the principal points lie on the viewing planeside, and a principal point space with the cemented lens becomes short.This is not preferable because difficulty is encountered in the balanceof astigmatism or coma in particular.

As the upper limit of 30 to Condition (11) is exceeded, the curvature ofthe lens surface of the one positive lens on the eye point side growstight. This is not preferable because aberrations of peripheral lightbeams such as coma are more produced.

For the viewing optical system here, it is preferable to have a fieldstop or an image display device on the viewing plane position, andsatisfy the following condition (12).

30<tan⁻¹(Y2/f)<47  (12)

Here Y2 is the diagonal length of the field stop or image displaydevice, and

f is the focal length of the whole viewing optical system.

As the lower limit of 30 to Condition (12) is not reached, it causes thefield of view to become narrow. This is not preferable because of theinability to increase resolution from constraints on the eye's resolvingpower.

As the upper limit of 47 to Condition (12) is exceeded, it causes thefocal length of the whole viewing optical system to become shortrelative to the viewing plane. This is not preferable because there iscoma produced.

With the embodiments here, it is possible to obtain a viewing opticalsystem that is compact and adequate in terms of the eye point distanceand angle of field with well corrected aberrations (such as distortion,astigmatism and chromatic aberrations), and an imaging apparatus usingthe same, as described above.

Some examples of the invention are now explained with reference to thedrawings. Note here that each example is directed to the application ofthe viewing optical system to a viewfinder. In what follows, therefore,the viewing optical system will be explained with reference to theviewfinder.

FIG. 1 is illustrative in section along the optical axis of the opticalarrangement of the first example of the viewfinder.

The viewfinder according to the first example has a viewing opticalsystem O located between a viewing plane D where an object image is tobe formed and an eye point E.

The viewing optical system O is made up of, in order from a viewingplane D side, a front lens component Lf, a first lens component L1 and asecond lens component L2. The front lens component Lf here consists of aplano-convex positive lens that is planar on the viewing plane D side.The first lens component L1 consists of a cemented lens of adouble-concave negative lens and a double-convex positive lens. Thesecond lens component L2 consists of a double-convex positive lens.

An aspheric surface is used at the lens surface of the front lenscomponent Lf on the eye point side.

FIG. 2 is illustrative in section along the optical axis of the opticalarrangement of the second example of the viewfinder.

The viewfinder according to the second example has a viewing opticalsystem O located between a viewing plane D where an object image is tobe formed and an eye point E.

The viewing optical system O is made up of, in order from a viewingplane D side, a front lens component Lf, a first lens component L1 and asecond lens component L2. The front lens component Lf here consists of aplano-concave negative lens that is planar on the viewing plane D side.The first lens component L1 consists of a cemented lens of adouble-concave negative lens and a double-convex positive lens. Thesecond lens component L2 consists of a double-convex positive lens.

An aspheric surface is used at the lens surface of the front lenscomponent Lf on the eye point side.

FIG. 3 is illustrative in section along the optical axis of the opticalarrangement of the third example of the viewfinder.

The viewfinder according to the third example has a viewing opticalsystem O located between a viewing plane D where an object image is tobe formed and an eye point E.

The viewing optical system O is made up of, in order from a viewingplane D side, a front lens component Lf, a first lens component L1 and asecond lens component L2. The front lens component Lf here consists of aplano-convex positive lens that is planar on the viewing plane D side.The first lens component L1 consists of a cemented lens of adouble-concave negative lens and a double-convex positive lens. Thesecond lens component L2 consists of a positive lens having a tightlyconvex surface on the eye point side.

An aspheric surface is used as the lens surface of the front lenscomponent Lf on the eye point side.

FIG. 4 is illustrative in section along the optical axis of the opticalarrangement of the fourth example of the viewfinder.

The viewfinder according to the fourth example has a viewing opticalsystem O located between a viewing plane D where an object image is tobe formed and an eye point E.

The viewing optical system O is made up of, in order from a viewingplane D side, a first lens component L1 and a second lens component L2.The first lens component L1 here consists of a cemented lens of adouble-concave negative lens and a double-convex positive lens, and thesecond lens component L2 consists of a double-convex positive lens.

To enable diopter to be corrected, the viewing optical system O may bedesigned such that the whole or a part of it is movable. When a part ofthe optical system is designed to be movable, the immovable or fixedportion has a dustproof effect on the viewing plane D. There may furtherbe a cover glass provided on the eye point side. Note here that when adisplay device such as a liquid crystal display device LCD or an organicEL device is used on the viewing plane D, the above viewfinder may beused as an electronic viewfinder.

Numerical data on Examples 1, 2, 3 and 4 will be set out below togetherwith the values of all the conditions.

Referring to the numerical data on and the values of Examples 1, 2, 3and 4, r is the radius of curvature of each lens surface, d is thesurface-to-surface space of each lens, n is the refractive index of eachlens, and v is the Abbe constant of each lens, with r for the imageplane being indicative of the radius of curvature. Note here thataspheric configuration is given by the following formula where x is anoptical axis provided that the direction of travel of light is positiveand y is a direction orthogonal to the optical axis.

x=(y ² /r)/[1+{1−(K+1)(y/r)²}^(1/2) ]+A2y ² +A4y ⁴ +A6y ⁶ +A8y ⁸ +A10y¹⁰

where r is a paraxial radius of curvature, K is a conic coefficient, andA2, A4, A6, A8 and A10 are the second-, fourth-, sixth-, eighth- andtenth-order aspheric coefficients, respectively.

In the numerical data, E±N (N is an integer) indicates ×10^(±N).

Numerical Example 1 in mm

Surface Data Effective Surface No. r d n νd Diameter 1 (Viewing ∞variable 17.75 Plane) 2 ∞ 2.00 1.52542 55.78 17.24 3 (Aspheric) 17.8724.04 17.07 4 −17.090 2.14 1.80518 25.42 17.25 5 26.464 10.12 1.8348142.71 20.94 6 −22.094 2.00 23.38 7 46.257 6.24 1.64000 60.08 26.80 8−40.996 variable 22.49 9 (Virtual ∞ Stop)

Aspheric Coefficient 3rd Surface K=0, A2=−4.26E-02, A4=1.63E-04

Amount of Change Diopter (m⁻¹) +1 −1 −3 d1 5.88 4.88 3.88 d8 22.00 23.0024.00

Various Data Diopter (m⁻¹) +1 −1 −3 Angle of 43.55° 44.19° 44.70° FieldTotal Length 32.43 31.43 30.43 Entrance 10224.98 −513.41 −251.68 PupilPosition

Focal Length: 22.1 Front Principal Point Position: 16.72 Rear PrincipalPoint Position: −0.39 Object Height: 8.88 Focal Length of the LensesFront Lens Component: 65.23 First Lens Component: 282.45 First LensComponent Negative Lens: −12.62 First Lens Component Positive Lens:15.93 Second Lens Component: 34.93 Numerical Example 2 in mm

Surface Data Effective Surface No. r d n νd Diameter 1 (Viewing ∞variable 17.75 Plane) 2 ∞ 4.53 1.52542 55.78 15.53 3 (Aspheric) 11.3165.45 13.67 4 −58.801 2.00 1.80518 25.42 14.27 5 24.592 7.56 1.6031160.64 15.03 6 −23.063 2.00 16.59 7 148.263 7.29 1.51633 64.14 16.83 8−18.350 variable 16.84 9 (Virtual ∞ Stop)

Aspheric Coefficient 3^(rd) Surface K=0, A2=−1.69E-02, A4=7.12E-05

Amount of Change Diopter (m⁻¹) +1 −1 −3 d1 6.66 4.76 3.06 d8 21.10 23.0024.70

Various Data Diopter (m⁻¹) +1 −1 −3 Angle of 31.67° 33.48° 35.02° FieldTotal Length 35.49 33.59 31.89 Entrance 36.20 36.99 38.07 Pupil Position

Focal Length: 30.7 Front Principal Point Position: 25.01 Rear PrincipalPoint Position: 16.82 Object Height: 8.88 Focal Length of the Lenses

Front Lens component: −34.85

First Lens Component: 138.28 First Lens Component Negative Lens: −21.31First Lens Component Positive Lens: 20.99 Second Lens Component: 32.10Numerical Example 3 in mm

Surface Data Effective Surface No. r d nd νd Diameter 1 (Viewing ∞variable 11.15 Plane) 2 ∞ 6.50 1.52542 55.78 11.24 3 (Aspheric) 8.4552.68 11.49 4 −16.092 1.20 1.80518 25.42 11.72 5 16.054 11.14 1.8160046.62 22.00 6 −17.911 1.53 16.31 7 1023.850 5.65 1.51633 64.14 15.98 8−20.220 variable 15.70 9 (Virtual ∞ Stop)

Aspheric Coefficient 3rd Surface K=0, A2=−8.60E-02, A4=1.47E-04

Amount of Change Diopter (m⁻¹) +1 −1 −3 d1 2.30 1.50 0.75 d8 22.20 23.0023.75

Various Data Diopter (m⁻¹) +1 −1 −3 Angle of 31.53° 31.61° 31.61° FieldTotal Length 30.99 30.19 29.44 Entrance −76.17 −66.15 −59.21 PupilPosition

Focal Length: 19.3 Front Principal Point Position: 17.39 Rear PrincipalPoint Position: −2.30 Object Height: 5.58 Focal Length of the Lenses

Front Lens component: 35.45

First Lens Component: 86.83 First Lens Component Negative Lens: −9.82First Lens Component Positive Lens: 12.17 Second Lens Component: 40.75Numerical Example 4 in mm

Surface Data Effective Surface No. r d nd νd Diameter 1 (Viewing ∞variable 17.74 Plane) 2 −89.444 1.50 1.80518 25.42 17.72 3 18.127 7.971.51633 64.14 18.09 4 −18.300 19.07 19.06 5 50.973 6.64 1.48749 70.2316.89 6 −28.519 variable 16.05 7 (Virtual ∞ Stop)

Amount of Change Diopter (m⁻¹) +1 −1 −3 d1 7.12 5.22 3.42 d6 21.10 23.0024.80

Various Data Diopter (m⁻¹) +1 −1 −3 Angle of 32.64° 32.90° 32.98° FieldTotal Length 42.30 40.40 38.60 Entrance 183.06 272.67 513.20 PupilPosition

Focal Length: 30.7 Front Principal Point Position: 24.53 Rear PrincipalPoint Position: −2.16 Object Height: 8.88 Focal Length of the Lenses

First Lens component: 141.18

First Lens Component Negative Lens: −18.60 First Lens Component PositiveLens: 19.06 Second Lens Component: 38.57

Condition Ex. 1 Ex. 2 Ex. 3 Ex. 4 (1) −0.8 −1.9 −0.8 −2.9 (2) 6.0~7.514.8~15   1.3~0.2 1.0~2.8 (3) 3.6~3.8 0.9~1.1 7.3~7.7 5.0~5.0 (4) −0.50−0.37 0.38 −0.48 (5) 1.80518 1.80518 1.80518 1.80518  (5)′ 1.834811.60311 1.81600 1.51633 (6) 0.03 0.20 0.011 0.29 (7) 44.0 33.0 31.6 32.6(8) 23.0 23.0 23.0 23.0 (9) 22.1 30.7 19.2 30.7 (10)  0.37 0.20 0.290.19 (11)  1.54 1.05 2.11 1.25 (12)  16.58 1.28 1.04 3.54

Aberration diagrams for Examples 1, 2, 3 and 4 are presented in FIGS. 5,6, 7 and 8, respectively, wherein SA, AS, DT, CC, DZY, FNO and FIY areindicative of spherical aberrations, astigmatism, distortion, chromaticaberrations, coma, an F-number and an image height, respectively.

FIG. 9 is illustrative of the arrangement of a digital camera that isone exemplar of the inventive imaging apparatus. In FIG. 9, referencenumeral 10 is a digital camera that is an imaging apparatus comprisingan imaging optical system 1, a filter 2, an imaging device 3, acontroller 4, a built-in memory 5, an electronic viewfinder 6 and aninterface 7.

In the above imaging apparatus, the imaging optical system 1 is built upof a plurality of optical elements (such as lenses). Light coming out ofan object is collected by this imaging optical system 1, and an objectimage is formed at this light collection position. And at this lightcollection position the imaging device 3 (light receiving plane) such asa CCD is located. The imaging device 3 is made up of an array ofregularly arranged photoelectric elements. To prevent the moiréphenomenon, the filter 2 having a low-pass effect is located between theimaging optical system 1 and the imaging device 3. There may also be aninfrared cut filter provided to cut off infrared light.

A light beam incident onto the imaging device 3 is converted by thephotoelectric elements into electric (image) signals. The electricsignals are entered in the controller 4 where signal processing such asgamma correction and image compression is applied to the electricsignals. The electric signals to which signal processing has beenapplied are sent out to a personal computer 9 or the like via thebuilt-in memory 5 and interface 7.

The electronic viewfinder 6 is made up of an illumination system, animage display device (not shown in FIG. 9), an eyepiece optical system(eyepiece lens), and so on. The inventive viewing optical system O isused for the eyepiece optical system here, and an image display deviceis located on the viewing plane D. This image display device iscontrolled by the controller 4. The electronic viewfinder 6 of sucharrangement enables the viewer to view an image taken, or being taken,of an object. Image data may be forwarded from the built-in memory 5 toan auxiliary memory 8. On the other hand, the same image data may beforwarded from the interface 7 to the personal computer 9.

FIG. 10 is illustrative of the arrangement of a silver-halide camera towhich the inventive imaging apparatus is applied. As shown in FIG. 10, asilver-halide camera 20 comprises an imaging optical system 11, a film12, an objective lens 13, an imaging device 14 such as a CCD, a firstcontroller 15, and a second controller 16. Further, there are a built-inmemory 5 and an electronic viewfinder 6 provided as is the case with thedigital camera of FIG. 9. Note here that the imaging optical system 11and the objective lens 13 are different optical systems, as shown.

With the silver-halide camera 20 shown in FIG. 10, a light beam comingout of an object is collected by the imaging optical system 11, and anobject image is formed on this light collection position (first lightcollection position). The film 12 is located at the first lightcollection position. A light beam coming out of the object is collectedby the objective lens 13, and an object image is formed at this lightcollection position (second light collection position). The imagingdevice 14 such as a CCD is located at the second light collectionposition. The imaging device 14 is made up of an array of regularlyarranged photoelectric elements.

A light beam incident onto the imaging device 14 is converted by thephotoelectric elements into electric signals (image signals). Theelectric signals are then entered in the first controller 15 wheresignal processing such as gamma correction and image compression isapplied to them. The electric signals to which signal processing hasbeen applied are sent out to the image display device. As describedabove, the electronic viewfinder 6 is constructed of an illuminationsystem, an image display device, an eyepiece optical system (eyepiecelens), and so on. The inventive viewing optical system O is used for theeyepiece optical system here. Via the electronic viewfinder 6, theviewer can view an object being taken of an object.

On the other hand, the user (viewer) can view the taken images, usinginformation or the like stored in the built-in memory 5. Such control isimplemented by the first controller 15.

For the purpose of controlling the imaging optical system 11, there isthe second controller 16 provided. The second controller 16 lets theimaging optical system 11 implement operations such as zooming andfocusing. Information for zooming, focusing or the like is recognized bythe first controller 15 based on signals from the second controller 16.By virtue of this recognition, the first controller 15 can work andadjust the image to be displayed on the image display device inconformity with the taking angle of view (zooming). On the basis ofinformation for focusing or the like, the range of the images displayedon the display device may be corrected (parallax correction). Signalsfrom the first controller 15 may also be sent out to the built-in memory5 or an interface (not shown). Then, these signals (information) may beproduced out to a personal computer or the like via the interface.

An optical path-splitting device may be located between the imagingoptical system 11 and the film 12. A light beam out of the object may beguided to the finder via that optical path-splitting device to form anobject image on the imaging device 14. And viewing may be implemented onthe basis of this object image. In this case, it is not necessary to usethe objective lens 13.

1. A viewing optical system positioned between a viewing plane as avirtual surface and an eye point, characterized by comprising, in orderfrom said viewing plane side, a cemented lens in which at least onenegative lens and at least one positive lens are cemented together, andone positive lens, with satisfaction of the following condition (1):−8<r3/f<−0.2  (1) where r3 is a radius of curvature of a lens surfacepositioned in said cemented lens and nearest to said viewing plane side,and f is a focal length of the whole viewing optical system.
 2. Theviewing optical system according to claim 1, characterized by satisfyingthe following condition (2):0°<εh<20°  (2) where εh is an exit angle (°) of a farthest off-axischief ray on said viewing plane provided that said farthest off-axischief ray is the outermost of off-axis chief rays that intersect anoptical axis of said viewing optical system at a given position that is20 mm spaced away from a lens surface located in said viewing opticalsystem and nearest to said eye point side toward the eye point side. 3.The viewing optical system according to claim 1, characterized bysatisfying the following condition (3):0<Enx/Y1<40  (3) where Enx is a distance from said viewing plane to anentrance pupil, and Y1 is a height of a given off-axis chief ray on saidviewing plane, provided that said given off-axis chief ray is defined bya chief ray corresponding to an angle of field of 30° of off-axis chiefrays that intersect an optical axis of said viewing optical system at agiven position that is 20 mm spaced away from a lens surface located insaid viewing optical system and nearest to said eye point side towardsaid eye point side.
 4. The viewing optical system according to claim 1,characterized by satisfying the following condition (4):−0.68<f12/f<−0.15  (4) where f12 is a combined focal length of lensesbetween said viewing plane and said negative lens, and f is a focallength of the whole imaging optical system.
 5. The viewing opticalsystem according to claim 1, characterized by satisfying the followingcondition (5) and (5)′:1.7<n<2.2  (5)1.7<n′<2.2  (5)′ where n is a refractive index of a lens located in saidcemented lens and nearest to said viewing plane side, and n′ is arefractive index of a lens located in said cemented lens and nearest tosaid eye point side.
 6. The viewing optical system according to claim 1,characterized by satisfying the following condition (6):|n−n′|<0.15  (6) where n is a refractive index of a lens located in saidcemented lens and nearest to said viewing plane side, and n′ is arefractive index of a lens located in said cemented lens and nearest tosaid eye point side.
 7. The viewing optical system according to claim 1,characterized by satisfying the following condition (7):13 mm<EP<40 mm  (7) where EP is an eye point distance that is a distancein mm from a lens surface located in said viewing optical system andnearest to said eye point side to said eye point.
 8. The viewing opticalsystem according to claim 1, characterized by satisfying the followingcondition (8):13.5 mm<f<45 mm  (8) where f is a focal length in mm of the wholeviewing optical system.
 9. The viewing optical system according to claim1, characterized by satisfying the following condition (9):0.08<tan θ×EP/f<1.6  (9) where θ is a maximum angle of field, EP is saideye point distance, and f is a focal length of the whole viewing opticalsystem.
 10. The viewing optical system according to claim 1,characterized by satisfying the following conditions (10) and (11):0.85<f1/f<3  (10)0<(r−r′)/(r+r′)<30  (11) where f1 is a focal length of said one positivelens, f is a focal length of the whole viewing optical system, r is aradius of curvature of a lens surface of said one positive lens on saidviewing plane side, and r′ is a radius of curvature of a lens surface ofsaid one positive lens on said eye point side.
 11. A viewfinder asrecited in claim 1, characterized in that there is a field stop or animage display device located on a position of said viewing plane, withsatisfaction of the following condition (12):30<tan⁻¹(Y2/f)<47  (12) where Y2 is a diagonal length of said field stopor said image display device, and f is a focal length of the wholeviewing optical system.
 12. An imaging apparatus, characterized bycomprising an imaging device, an image display device adapted to displayan image, a controller adapted to convert image information obtainedfrom said imaging device into signals displayable on said image displaydevice, and a viewfinder adapted to guide an image displayed on saidimage display device to a viewer's eye, wherein the viewing opticalsystem according to claim 1 is used for said viewfinder.